1. Field of the Invention
Apparatuses consistent with the present invention relate to a demodulation circuit for use in a receiver, and more particularly, to a demodulation circuit for a receiver, which has a small hardware size and consumes low power.
2. Description of the Related Art
Generally, a heterodyne receiver does not directly modulate a carrier frequency signal to a baseband frequency signal. Rather, the heterodyne receiver has a system which converts an incoming signal to an intermediate frequency (hereinafter referred to as “IF”) signal between a carrier frequency and a baseband frequency, for processing. Such heterodyne receiver can down-convert the IF signal to the baseband signal, demodulate the baseband signal and then digitize the demodulated signal. Otherwise, the heterodyne receiver can directly digitize the IF signal to a digital signal and then demodulate the digitized signal. The latter method is called an IF direct sampling.
FIG. 1 illustrates a block diagram of a receiver using an IF direct sampling method in accordance with a related art. As shown in FIG. 1, an IF direct sampling type receiver in accordance with a related art includes an RF tuner 10, a “Surface Acoustic Wave filter” 15 (hereinafter referred as “SAW filter”), an IF amplifier 20, an A/D converter 25, a demodulation circuit 30 and a signal processing block 50.
The RF tuner 10 receives an RF signal via an antenna in synchronization with a frequency of the RF signal and converts the received RF signal to an IF signal.
The SAW filter 15 is a communication filter using mechanical vibrations of a piezo-electric substrate. In case that an electrical signal is input to the SAW filter 15, surface acoustic wave occurs over the piezo-electric substrate and it is converted to an electrical signal again. At this time, if a frequency of the surface acoustic wave and a frequency of an incoming electrical signal are different, the incoming signal can not be transferred. That is, only the desired frequency signals can pass through the SAW filter 15.
The IF amplifier 20 amplifies an output signal of the SAW filter 15 so as to keep the level of a signal to be input to the A/D converter 25 constant.
The A/D converter 25 converts the IF signal to the digital signal and receives a clock signal at a frequency equal to 4 times the baseband sample frequency from a clock generator during the digital signal conversion.
The demodulation circuit 30 for demodulating the digitalized IF signal includes a pair of mixers 35, a pair of “low-pass filters” 40 (hereinafter referred to as “LPF”), and a pair of down sampling units 45.
The mixer 35 down-converts the IF signal to the baseband level by combining the IF signal output from the A/D converter 25 with a sine wave or a cosine wave at the same frequency as the IF signal.
A pair of the LPFs 40 filters the down-converted IF signal to eliminate high frequency signals and eliminates noises in a stopband. At this time, the LPF 40 should operate four times faster than general LPFs since the sampled signals input to the LPFs 40 have a frequency four times greater than the baseband signals.
The down sampling unit 45 selects desired samples out of the IF signals output from the LPFs 40, performs 4 times of sampling operations in one period and outputs “Inphase” signals (hereinafter referred as “I”) and “Quardrature” signals (hereinafter referred as “Q”).
The receiver described above down-converts the IF signals digitized by the A/D converter 25 to the baseband signals using a sine and a cosine waves to produce the I signals and Q signals. Accordingly, a pair of the mixers 35 are needed to receive the sine and cosine waves and a pair of the LPFs 40 are needed to eliminate noises in the signals down-converted to the baseband. Therefore, the receiver is disadvantageous in that it has a large hardware size because it needs pairs of the mixers and the LPFs, and the cost is high due to the large hardware size.
Further, in the receiver described above, the LPFs 40 should operate four times faster than general LPFs to filter the IF signal since it is sampled at a sampling rate four times greater than the baseband signal frequency. Accordingly, the receiver has a drawback in that it consumes much power since the LPFs 40 operate at high speed.
Accordingly, it is demanded that the receiver should be realized in a small size by reducing the number of mixers 35 and LPFs 40 provided in pairs, respectively in the receiver in accordance with the related art. Further, it is demanded that power consumption be reduced by lowering the operational speed of the LPF 40 when the LPF 40 processes the high frequency IF signals produced by the A/D converter 25.